Not so much a book but a series, namely the Discworld series by Terry Pratchett. I really can't pick a favourite out of them though. I saw the Hogfather TV adaption and picked up the book out of curiosity. That got me hooked onto the series. It's a shame there won't be any more of them now

I absolutely love Terry Pratchett's work, one the very best authors ever, I still have one of his final books left to read, but I don't want to pick it up because once I've read it there won't be any more to look forward to.